Liquid level controllers for controlling the level of liquid in a storage vessel are generally known in the art. As is known, such liquid level controllers typically use a pneumatic pilot valve. The pneumatic pilot valve typically includes a pneumatic inlet, a pneumatic outlet, and an exhaust. Such pneumatic pilot valves are typically connected to a pneumatic source, such as shop air.
However, in some process environments, a source of shop air is not available, and in those process environments the pneumatic pilot valve is connected to another pneumatic source, such as the pressurized process medium in the process environment. Typically, the process environment is an industrial manufacturing, processing and/or storage facility such as a chemical plant, a refinery and/or a shipping terminal.
Many pneumatic pilot valves typically include a first valve, a second valve, and a shiftable thrust pin. The thrust pin shifts between a first position in which the first valve is closed and a second position in which the second valve is closed. Such valves typically exhaust a measurable quantity of the pneumatic source when the thrust pin shifts between a first position and a second position. In the event the pneumatic source is shop air, the periodic loss of a measurable quantity of shop air is not a concern. However, when pneumatic pilot valves are used in some of the process environments discussed above where shop air is not available, the venting of a measurable quantity of the process medium can have measurable monetary and environmental implications.